This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Most raw natural gas extracted from the Earth is primarily methane (CH4) with smaller amount of other hydrocarbons and varying degrees of different “contaminants.” The methane component, as a low molecular weight hydrocarbon, is typically the desirable component within the raw natural gas. Applications for methane recovered from raw natural gas may include fuel for vehicles, power generation, and so on. Moreover, in many cities, such CH4 is piped into homes for domestic heating and cooking purposes. In this context, methane is usually known as natural gas.
The demand to provide effective techniques to separate and remove contaminants from raw natural gas to give purified natural gas or methane has significantly increased. The contaminants and impurities in raw natural gases may include acid gases such as carbon dioxide (CO2), hydrogen sulfide (H2S), mercaptans, and the like. The contaminants may also include nitrogen (N2), helium (He), water vapor, liquid water, mercury, and so forth. Such contaminants and impurities may lead to equipment malfunction, production failure, product contamination, among other detrimental production issues. For example, CO2 contaminant when combined with water may create a corrosive form of carbonic acid. Additionally, for example, at CO2 concentrations of more than 2%, the presence of the CO2 may reduce the BTU value of the natural gas and lower the economic viability of the natural gas.
To remove CO2 and other acid gases, the natural gas may be subjected to cryogenic distillation. Distillation is a widely used way of effecting relative-volatility based separations. A cryogenic distillation system may provide for separating mixtures based on their relative volatility at cryogenic temperatures. In some cryogenic distillation systems, one or more components may solidify. Indeed, in the cryogenic distillation, the operating temperature, pressure, and component concentrations may lead to solidification of the CO2. Therefore, certain purification systems may control such solidification. For instance, the system may provide for a freezing section in a cryogenic distillation column in which CO2 solidification is permitted and managed. If so, solidification of CO2 outside of the freezing section is typically undesirable. Such solidification of CO2 outside of the freezing section may be more prone to occur during startup of the distillation system when disturbances are more likely during initial conditions and because of the transient and non-linear behavior of startup. The undesired solidification, e.g., CO2 solidification outside of the freezing section, may occur in the distillation column or elsewhere in the distillation column system, such as in the overhead system of the distillation column.
In general, during normal operations of the cryogenic distillation column, CO2 contaminant, among other contaminants, may be separated and removed from the raw natural gas to produce a purified methane gas product. To startup the cryogenic distillation column, prior to steady state or normal operating conditions, various startup techniques may be implemented. For instance, the startup may be “assisted” in which solidification inhibitors, and/or a clean methane flow for liquid reflux, are utilized until normal operation is reached, while avoiding solidification in undesirable locations. Unfortunately, the solidification inhibitors and the clean methane gas either impose additional costs or may not be readily available.
In the worldwide natural-gas production industry where hundreds of trillions of standard cubic meters of raw natural gas are extracted from the Earth and processed, there remains an ongoing need to continuously improve such extraction and processing techniques. In particular, there remains a need for improved cryogenic distillation systems for separating contaminants from raw gas streams.